Centralize testing key/cert constants in test/rubygems/pem_utilities.rb

[junaruga]

This commit refactors testing key/cert constants in a more consistent way.

The key/cert constants were duplicated across the files, and there was a
different constant naming *_PATH vs *_FILE.

Create test/rubygems/pem_utilities.rb moving .pem methods/constants from
test/rubygems/helper.rb to manage testing key/cert constants in one file.
test/rubygems/local_ssl_server_utilities.rb, used by both RubyGems and
Bundler, also uses these constants, so the new pem_utilities.rb must not
depend on test-unit.

In test/rubygems/pem_utilities.rb, added the following features compared to
test/rubygems/helper.rb.

• Add pem_file method for combined cert+key PEM files
• Rename helper methods from key_path to key_file, from cert_path to
  cert_file, to consistently use the *_FILE suffix for constants.
• Align test/rubygems/*.pem file names and constants with the pattern
  <algorithm>_<name>. If the <algorithm> is RSA 2048 bits, there is no
  <algorithm>_ prefix as a default algorithm.
• Rename EXPIRED_PUBLIC_CERT to EXPIRED_CERT. CERT is only public. There is no private
  cert file.
• Update local_ssl_server_utilities.rb to use constants directly,
  removing unused methods.
• Handle OpenSSL::PKey::PKeyError in load_key for unsupported
  algorithms on old OpenSSL versions

Assisted-by: Claude:claude-opus-4-6[1m]

What was the end-user or developer problem that led to this PR?

This PR is to refactor testing constant managements addressed on the #9697. In the current testing constants, it was not convenient to add more ML-DSA-65 tests in test/rubygems/test_gem_security*.rb with inconsistent constants such as _PATH vs _FILE, and duplicated constants across testing files, and inconsistent test/rubygems/*.pem file and constant naming patterns.

What is your fix for the problem, implemented in this PR?

This PR refactors testing key/cert constants in a more consistent way, managing constants in in one file, and aligning test/rubygems/*.pem file names and constants with the pattern <algorithm>_<name>, removing duplicated and unused methods/constants.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[junaruga]

@hsbt Could you review this PR? This was found on for #9697, and this refactors the code. After this PR is merged, I can rebase #9697 on the latest master branch.

The Ubuntu ruby-4.0 CI case failed with a random internet error. I don't think this failure is related to this PR.
https://github.com/ruby/rubygems/actions/runs/31034718272/job/92404006797?pr=9768#step:12:117

[junaruga]

This line is mistake. The code was originally intended as below. However, it seemed that ALTERNATE_CERT was used as CHILD_CERT in one part. So, I replaced the one part ALTERNATE_CERT with CHILD_CERT.

ALTERNATE_CERT = load_cert "alternate"